Use of a duplex stainless steel in a phosphoric acid production system

ABSTRACT

The present disclosure relates to the use of a duplex stainless steel as heat exchanger material in a phosphoric acid production system using the wet method. The steel has the following composition in percent by weight: C max 0.03 Si max 0.5 Mn max 3 Cr 26-29 Ni 4.9-10 Mo 3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3 balance Fe and normal occurring impurities.

FIELD OF INVENTION

The present invention relates to a heat exchanger for use in an evaporator in a phosphoric acid production system. More specifically, it relates to a metallic heat exchanger comprising a tube of a metallic material, said heat exchanger intended to be used in an evaporator use in a phosphoric acid production be means of the wet method. Furthermore, the present invention relates to the use of a duplex stainless steel in environments containing phosphoric acid.

BACKGROUND

Phosphoric acid (H₃PO₄) can be produced by two different methods, commonly known as the wet method, in which phosphate ore is used to produce the phosphoric acid, and the thermal or hot method, in which elemental phosphorus is used to produce the phosphoric acid. The majority of the phosphoric acid used today is produced by means of the wet method since it is less costly than the thermal process. The wet method phosphoric acid is for example commonly used in fertilizer production. The thermal phosphoric acid is of a much higher purity and is for example used in the manufacture of high grade chemicals, pharmaceuticals, detergents and food products.

The wet method comprises reacting diluted sulphuric acid (H₂SO₄) with naturally occurring phosphate rock (generally consisting of calcium phosphate Ca₃(PO₄)₂) thereby producing a calcium sulphate slurry and phosphoric acid, which are separated by filtration. The acidic filtrate is recycled to the reactor to concentrate the P₂O₅ content of the acid produced. The temperature is generally between 70-90° C. Before purification, the produced crude acid is concentrated and clarified. An additional step in which precipitates of sulphate arsenic and fluorosilicates are removed is often included prior to purification.

The corrosiveness of phosphoric acid during wet-process concentration is quite complex and is dependent on several influencing factors. The factor, which has the most significant impact, is the presence of impurities. For example, at a given concentration, the presence of fluorides, chlorides and dilute sulfuric acid in the process will increase the corrosivity of the acid.

Pure phosphoric acid is less corrosive than both sulfuric acid and hydrochloric acid. Thus, standard stainless steels, such as AISI 316L and 317L, are sufficient materials for construction equipment when the material is in contact with pure phosphoric acid. However, the wet method phosphoric acid invariably contains impurities, which are derived from the phosphate rock from which the acid is produced. The concentration of fluoride and chloride during the wet method varies greatly from plant to plant depending on the origin of the phosphate rock, i.e. the composition of the phosphate rock. The acid also contains other ions, such as Fe³⁺, which affect the corrosion properties. Fe³⁺ strongly contributes to the oxidizing potential of the acid and when present in sufficient amounts it therefore reduces corrosion of a stainless steel by facilitating the formation of a passive film on the steel surface. Thus, the process media is very complex and individual. This should be taken into consideration when selecting material for a tube of the heat exchanger in the evaporator since the tube will be in direct contact with the process media.

Moreover, the temperature can vary in the process and it is required to use the heat exchangers in the evaporator at high temperatures in order to increase the efficiency of the process. This also puts high demands on corrosion resistance of a material in contact with the process media.

Historically, the most widely used material for heat exchanger tubes to be used in the wet method has been graphite. However, the mechanical weakness and brittleness of graphite is a major drawback which often resulted in repeated problems with broken tubes and thereby loss of production. With the development of improved high-alloyed materials, metallic construction of heat exchangers has become more common and a preferred solution during the last decade.

The most widely used metallic material for evaporator tubes in the manufacture of phosphoric acid by the wet method today is an austenitic stainless steel with the following composition in percent by weight:

C max 0.02 Si max 0.7 Mn max 2 Cr  26-28 Mo   3-4 Ni  30-32 Cu 0.7-1.5 N max 0.1

balance Fe and normally occurring impurities.

This austenitic stainless steel is known under the standard UNS N08028. UNS N08028 generally performs very well as material for evaporation tubes. However, if the life time of a tube of a heat exchanger in the evaporator could be even longer, there would be less production loss due to shut downs for changing pipes.

Moreover, a duplex stainless steel known under standard UNS S32520 is used for construction of phosphoric acid storage tanks in phosphoric acid production plants. This duplex stainless steel has the following composition in percent by weight:

C max 0.030 Si max 0.80 Mn max 1.5 Cr   23-25 Mo   3-5 Ni  5.5-8 Cu  0.5-3.0 N 0.20-0.35

balance Fe and normally occurring impurities.

UNS S32520 has also been proposed for construction of vessels, piping, fittings and other proprietary devices in phosphoric acid production plants since it is considered to have good corrosion resistance in phosphoric acid production plant environments. To the best of the applicant's knowledge, this material has not yet been proposed as alternative material for heat exchangers but would probably be sufficient since it can be used in other parts of the plants which are exposed to similar conditions. However, a metallic material with even better corrosion resistance in the environment would probably reduce the number of shut downs for changing pipes and consequently improve production of a phosphoric acid production plant.

Furthermore, a nickel based material known under the name Hastelloy® G-30 has been proposed for phosphoric acid environments. This nickel based alloy comprises approximately max 0.03% C, max 0.8% Si, max 1.5% Mn, 29.5% Cr, max 5% Co, 5% Mo, 3% W, 15% Fe, 1.7% Cu and 0.9% Nb+Ti. The corrosion resistance of this material is very good in the phosphoric acid environment, but G-30 is very expensive as a result of the composition and is therefore not considered as a cost-effective material for use as heat exchanger material in a phosphoric acid production plant.

The object of the invention is therefore to, to a reasonable cost, improve life time of a heat exchanger for evaporation systems in phosphoric acid production systems using the wet method.

SUMMARY OF INVENTION

The above identified object is accomplished by utilizing a duplex stainless steel with the following composition in percent by weight:

C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

Balance Fe and Normal Occurring Impurities

as the tube material for the heat exchanger in the evaporator.

Impurities in the duplex stainless steel may result from the raw material used for production of the steel and/or be present in the steel as a result of the production method used. Examples of impurities are S, Al and Ca.

The duplex stainless steel used in accordance with the present invention has proven to have an increased corrosion resistance to environments containing phosphoric acid compared to the commonly used austenitic stainless steel UNS N08028. It is also believed that it has better corrosion resistance to the environment than UNS S32520.

It has further been established that the duplex stainless steel according to the invention performs very well at temperatures at least up to 110° C. in the intended environment. Since corrosion resistance is the most critical parameter for a tube to be used in the heat exchanger, the life time of the heat exchanger is prolonged by utilizing this duplex stainless steel.

The use of the duplex stainless steel is especially advantageous in phosphoric acid production systems using the wet method and wherein the process solution contains 30-80% H₃PO₄, up to 2000 ppm Cl⁻ and up to 2% F⁻.

Even though the object of the present invention is related to heat exchangers to be used in the evaporation during manufacturing of phosphoric acid, it is reasonable to believe that the duplex stainless steel identified above is also suitable for use in other applications subjected to environments containing phosphoric acid. Examples of such applications may for example be any application wherein phosphoric acid produced by means of the wet method is used to produce a final product as long as the duplex stainless steel described above also is suitable for use in the environment of the other components used to produce the final product and under the process conditions, such as temperature and pressure, required for the production of the final product. The duplex stainless steel is considered suitable as material at least for vessels, piping, fittings and proprietary devices in phosphoric acid production plants. The duplex stainless steel may also be used as construction material in fertilizer production plants for parts in contact with phosphoric acid containing media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the result of a corrosion test in phosphoric acid with three different concentrations.

FIG. 2 shows the iso-corrosion curve for 0.1 mm/year of the duplex stainless steel used according to the invention.

FIG. 3 shows the temperature dependence on the corrosion rate of the duplex stainless steel used according to the invention.

DETAILED DESCRIPTION

The duplex stainless steel used according to the present invention has the following composition in percent by weight:

C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

balance Fe and normal occurring impurities

The effect of the different alloying elements has been described in detail in US2003/086808 A1 and will therefore not be discussed further here.

The duplex stainless steel has a ferrite content of 40-65%. Furthermore, it has a well balanced composition such that both the ferrite and the austenite phase have high corrosion resistance as a result of the alloying elements being well distributed between the two phases. The PREW value of the alloy is at least 45, wherein PREW is [wt-% Cr]+3.3([wt-% Mo]+0.5[wt-% W])+16[wt-% N]. Preferably, the PREW value of each phase, i.e. ferrite and austenite, is at least 45. More preferably, the relationship [PREW_(austenite)]/[PRE_(ferrite)] is 0.9-1.15. The PRE value of the “weakest” phase (i.e. the one with the lowest PRE value and thereby the lowest corrosion resistance) will always limit the corrosion resistance of the alloy as a whole. Furthermore, the other phase will have an unnecessary high content of the alloying elements beneficial for the corrosion resistance, which in turn leads to a higher risk of deteriorated structure stability in the “stronger” phase. With a balanced PRE, an optimum of both the corrosion resistance and the structure stability is accomplished.

According to one preferred embodiment, the duplex stainless steel comprises max 1.2% Cu. According to another preferred embodiment, the duplex stainless steel comprises 0.5-3.5% Co. According to yet another preferred embodiment the duplex stainless steel comprises 26.5-28% Cr.

The proof strength and tensile strength, when in the form of a solution annealed seamless tube, of the duplex stainless steel used according to the present invention is listed in Table 1. These figures can for example be compared to UNS N08028 which has a minimum tensile strength at 100° C. of 510 MPa when in the form of a seamless tube. Clearly the mechanical strength of the duplex stainless steel used according to the present invention is much higher than the conventionally used UNS N08028.

TABLE 1 Temperature Proof strength R_(p0,2) Tensile strength R_(m) [° C.] [MPa] [MPa] 50 min. 645 min. 900 100 min. 600 min. 850 150 min. 560 min. 840

According to a preferred embodiment, the duplex stainless steel has the following nominal composition in percent by weight:

C max 0.03 Si 0.3 Mn 1 P max 0.035 S max 0.01 Cr 27 Ni 6.5 Mo 4.8 Co 1 N 0.4

balance Fe and normally occurring impurities.

Example 1

Test samples in the form of tube-halves were produced from steels with the following composition in percent by weight:

C 0.013 Si 0.37 Mn 0.89 P 0.015 S 0.0005 Cr 26.45 Ni 6.45 Mo 4.77 Co 0.97 N 0.40

balance Fe and normally occurring impurities.

General corrosion, according to ASTM G 31-72 rev 2004, was performed at 100° C. in commercial phosphoric acid of two concentrations and 70% synthetic H₃PO₄ with 4% H₂SO₄ and 0.45% Fe³⁺. The compositions of the different phosphoric acids are listed in Table 1.

TABLE 1 The concentrations of the test solutions P₂O₅ H₃PO₄ Cl⁻ F⁻ Test solution (w-%) (w-%) (ppm) (w-%) “Strong” commercial H₃PO₄ 54 ~75 ~460 0.32 “Weak” commercial H₃PO₄ 39 ~54 ~1700 1.8 Synthetic H₃PO₄ ~50 70 600 0.7

All corrosion tests were performed using double samples. The result is shown in Table 2 and illustrated in FIG. 1 wherein the mean value of the result of the two samples is shown. It is clear that the duplex stainless steel has a lower corrosion rate than UNS N08028 in all of the tested phosphoric acid concentrations.

TABLE 2 Corrosion rate (mm/year) Duplex stainless steel according Test solution UNS N08028 to the invention “Strong” commercial H₃PO₄ 0.057/0.054 0.051/0.054 “Weak” commercial H₃PO₄ 0.072/0.069 0.053/0.053 Synthetic H₃PO₄ 0.060/0.061 0.039/0.046

Example 2

Test samples in the form of tube halves were produced of an alloy with the following composition in percent by weight:

C 0.014 Si 0.26 Mn 1.00 P 0.011 S <0.0005 Cr 26.68 Ni 6.40 Mo 4.72 Co 0.95 N 0.38

balance Fe and normally occurring impurities.

Furthermore, test samples of the alloy UNS N08028 in the form of tube halves were produced for comparison.

General corrosion testing according to ASTM G 31-72 rev 2004, was performed at 100° C. in synthetic phosphoric acid with the following composition:

H₃PO₄   70% H₂SO₄   4% Fe³⁺ 0.45% Cl⁻ 300-1200 ppm F⁻ 0.1-1.2%

The result in mm/year is shown in Table 3 wherein every value is a mean value of two samples. The iso-corrosion curve for 0.1 mm/year is shown in FIG. 2.

It is clear from the results that the duplex stainless steel according to the present invention has a good resistance to phosphoric acid in different chloride and fluoride concentrations.

TABLE 3 F⁻ (%) 0.1 0.3 0.5 0.7 0.8 0.9 1.0 1.2 Cl⁻ 300 0.088 (ppm) 500 0.058 0.077 0.086 0.084 0.089 0.061 700 0.100 0.085 0.330 800 0.080 0.074 0.209 0.063 1000 0.070 0.079 0.082 0.077 0.140 1200 0.077 0.080 0.261 0.075 0.277 0.076

Example 3

Test samples in the form of tube halves were produced of an alloy with the following composition:

C 0.015 Si 0.29 Mn 0.95 P 0.012 S 0.0006 Cr 26.62 Ni 6.42 Mo 4.73 Co 0.98 N 0.38

balance Fe and normally occurring impurities.

General corrosion testing, according to ASTM G 31-72 rev 2004, was performed in 70% H₃PO₄, 4% H₂SO₄, 0.45% Fe³⁺ at different concentrations of Cl⁻ and F at 100° C. to verify the iso-corrosion curve seen in FIG. 2 in the previous example. The different concentrations of Cl⁻ and F, as well as the result of the tests are shown in Table 4. The results correspond very well to the iso-corrosion curve in FIG. 2.

TABLE 4 Cl⁻ (ppm) 500 600 700 750 800 900 1000 1100 1200 F⁻ (%) 1.2 1 0.6 0.7 0.7 0.7 0.7 0.7 0.7 Average corrosion 0.09 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 (mm/year)

Example 4

Test samples in the form of tube halves were produced of an alloy with the following composition:

C 0.015 Si 0.29 Mn 0.95 P 0.012 S 0.0006 Cr 26.62 Ni 6.42 Mo 4.73 Co 0.98 N 0.38

balance Fe and normally occurring impurities.

Furthermore, test samples of the alloy UNS N08028 in the form of tube halves were tested for comparison.

General corrosion test, according to ASTM G 31-72 rev 2004 at 100° C., was performed in commercial phosphoric acid with the concentration 39% H₃PO₄ and approximately 1380 ppm Cr. The concentration of F was not analyzed in this case. The results are summarized in Table 5.

TABLE 5 Corrosion rate Duplex stainless steel according 0.025 mm/year to the invention UNS N08028 0.028 mm/year

Example 5

The temperature dependence of the general corrosion in synthetic phosphoric acid was investigated in the temperature range 80-110° C. The acid had the following composition:

H₃PO₄   70% H₂SO₄   4% Fe³⁺ 0.45% Cl⁻ 500 ppm F⁻  0.5%

Test samples in the form of tube-halves were produced from steels with the following composition in percent by weight:

C 0.013 Si 0.37 Mn 0.89 P 0.015 S 0.0005 Cr 26.45 Ni 6.45 Mo 4.77 Co 0.97 N 0.40

balance Fe and normally occurring impurities.

The results are listed in Table 6 and illustrated in FIG. 3. It is clear that the corrosion rate increases with increased temperature, especially over 100° C. However, the corrosion rates up to at least the tested 110° C. are not detrimental.

TABLE 6 Temp (° C.) 80 90 100 100 105 110 Average corrosion 0.02 0.05 0.08 0.23 0.55 0.55 rate (mm/year)

Example 6

Joining of the duplex stainless steel used according to the present invention to the conventionally used austenitic stainless steel UNS N08028 was tested in order to establish if it is possible to join the two materials without losing corrosion resistance in the weld. This was done to verify that UNS N08028 could be used as wall material in the heat exchanger with the duplex material as tube material, in the case such a solution would be desirable.

Tubes in the dimensions 19.05×1.65 mm were used. Girth welds were made using conventional TIG welding. General corrosion test, according to ASTM G 31-72 rev 2004 at 100° C., in synthetic phosphoric acid was performed. The composition of the acid is listed in Table 7. The corrosion rate was low and comparable to the corrosion rate of UNS N08028. It is therefore clear that the duplex stainless steel used according to the present invention can easily be joined with the commonly used UNS N08028.

TABLE 7 H₃PO₄   70% H₂SO₄   4% Fe³⁺ 0.45% Cl⁻ 500 ppm F⁻  0.5%

Example 7

A previous corrosion test has shown that UNS N08028 has better corrosion resistance than the duplex stainless steel UNS S32520. Hence, it is considered that the steel used according to the present invention is also better than UNS S32520 since it has been established above that the duplex stainless steel according to the invention has better corrosion resistance than UNS N08028. Hence, the life time of a heat exchanger tube in accordance with the present invention would be longer than the life time of a possible heat exchanger tube of UNS S32520.

The test was performed on samples taken from TIG-welded material. The tested compositions of UNS S32520 and UNS N08028 are shown in Table 8. UNS S32520 was welded using argon with 2% N₂ as shielding gas and with the filler material 25 9 4 N L (according to standard EN ISO 14343), whereas UNS N08028 was welded using essentially pure argon as shielding gas and with the filler material 27 31 4 Cu L (according to standard EN ISO 14343).

TABLE 8 UNS S32520 UNS N08028 C 0.015 0.009 Si not analyzed 0.48 Mn 1.030 1.77 P not analyzed 0.011 S 0.0003 not analyzed Cr 25.05 26.07 Ni 6.48 30.38 Mo 3.67 3.21 Cu 1.68 0.93 N 0.244 0.055 Fe Bal. Bal.

The general corrosion test, according to ASTM G 31-72 rev 2004, was performed at a temperature of 90° C. using a duration of 1+3+3 days. The phosphoric acid used had the following composition:

H₃PO₄ ~58% P₂O₅ ~42% Cl⁻ 620 ppm F⁻  1.8%

The result showed that UNS N08028 had a mean corrosion rate of 0.0626 mm/year and UNS S32520 had a mean corrosion rate of 0.0730 mm/year. From this test it is clear that UNS S32520 corrodes much faster than UNS N08028 and thus has a shorter service life in phosphoric acid environments containing impurities. 

1. Heat exchanger for use in environments containing phosphoric acid, said heat exchanger comprising at least one tube of a duplex stainless steel with the following composition in percent by weight: C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

balance Fe and normal occurring impurities.
 2. Heat exchanger according to claim 1 wherein the tube is a seamless tube.
 3. Heat exchanger according to claim 1 wherein the duplex stainless steel comprises 26.5-28% Cr.
 4. Heat exchanger according to claim 1 wherein the duplex stainless steel comprises max 1.2% Cu.
 5. Heat exchanger according to claim 1 wherein the duplex stainless steel comprises 0.5-3.5% Co.
 6. Heat exchanger according to claim 1 wherein [wt-% Cr]+3.3([wt-% Mo]+0.5[wt-% W])+16[wt-% N] is at least
 45. 7. Heat exchanger according to claim 1 wherein it is adapted to be in direct contact with a process solution containing phosphoric acid.
 8. Phosphoric acid production system comprising an evaporator having a heat exchanger, said heat exchanger comprising at least one tube of a duplex stainless steel with the following composition in percent by weight: C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

balance Fe and normal occurring impurities.
 9. Phosphoric acid production system according to claim 8 wherein the heat exchanger is adapted to be in direct contact with a process solution containing phosphoric acid.
 10. Phosphoric acid production system according to claim 8 wherein the tube is a seamless tube.
 11. Phosphoric acid production system according to claim 8 wherein the duplex stainless steel comprises 26.5-28% Cr.
 12. Phosphoric acid production system according to claim 8 wherein the duplex stainless steel comprises max 1.2% Cu.
 13. Phosphoric acid production system according to claim 8 wherein the duplex stainless steel comprises 0.5-3.5% Co.
 14. Phosphoric acid production system according to claim 8 wherein [wt-% Cr]+3.3([wt-% Mo]+0.5[wt-% W])+16[wt-% N] is at least
 45. 15. Use of a duplex stainless steel with the following composition in percent by weight: C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

balance Fe and normal occurring impurities in environments containing phosphoric acid.
 16. Use of a duplex stainless steel with the following composition in percent by weight C max 0.03 Si max 0.5 Mn max 3 Cr   26-29 Ni  4.9-10 Mo   3-5 N 0.35-0.5 B max 0.0030 Co max 3.5 W max 3 Cu max 2 Ru max 0.3

balance Fe and normal occurring impurities as a tube in a heat exchanger in environments containing phosphoric acid.
 17. Use according to claim 16 wherein the heat exchanger is a heat exchanger in an evaporator in a phosphoric acid production system using the wet method.
 18. Use according to claim 15 in environments containing 30-80% H₃PO₄, up to 2000 ppm Cl″ and up to 2% F″.
 19. Use according to claim 15 at temperatures up to 110° C.
 20. Use according to claim 15 in fertilizer production plants. 